In British Patent Specification 1520976 there is described a method of preparing silver halide emulsions wherein the silver halide crystals are of the twinned type. This method involves the formation of silver iodide seed crystals. A soluble silver salt and another halide are added to the silver iodide seed crystals. In British patent specification 1570581 it is shown that the silver iodide seed crystals formed are of the truncated bipyramidal hexagonal lattice habit. When soluble silver and other halide salts are added to the dispersion of the silver iodide seed crystals the silver iodide crystals act as sites for the epitaxial growth of the twinned silver halide crystals. Similar growth of twinned silver halide crystals is shown in British Patent Specification 1596602.
In the process as described in BP 1570581 and in BP 1596602 crystals of high iodide content are first formed. Silver halide crystals which have a high iodide content that is to say from 90 to 100 mole % iodide are predominantly of hexagonal lattice structure. Techniques for the preparation of silver iodide crystals predominantly of hexagonal lattice structure are well-known, and are for example described by B L Byerley and Hirsch, J. Phot. Sci., Volume 18, p 53 (1970). Such crystals have the shape of hexagonal pyramids or bipyramids. The basal faces of these pyramids comprise the lattice planes (0001). Silver iodide crystals of the hexagonal lattice structure are shown in FIG. 2 of British Pat. No. 1570581.
The disclosures of all documents referred to in this specification are incorporated by reference in their entirety.
In the process in step (b) as set out in No. 1570581 aqueous solutions of a silver salt and an alkali metal or ammonium bromide or chloride (or mixtures thereof) are added to the dispersion medium containing the silver iodide crystals which are predominantly of the hexagonal lattice structure, so that silver iodobromide (or iodochloride or iodochlorobromide) is precipitated. The mixed halide crystals precipitated are of the face centered cubic structure. These crystals incorporate silver iodide from the dissolving seed crystals up to a maximum of approximately 40 mole % of the total halide at a temperature of approximately 65.degree. C. However, during this step the first-formed silver iodide crystals dissolve and the silver iodide is incorporated into the growing face-centered cubic lattice crystals. Electron micrographs have revealed that in step (b), whilst no overall circumferential growth of the silver iodide crystals occurs, the face-centered cubic lattice type crystals of the halide being added in step (b) form and grow epitaxially on the basal faces of the silver iodide crystals formed in step (a). Epitaxial growth is possible between (0001) AgI faces and (111) AgBr or AgCl faces because both are hexagonally close-packed, homoionic lattice planes. It has been observed by electron microscopy that at least about 90% of the growing epitaxial crystals are twinned (recognized by the parallel striations characteristic of several twin planes intersecting the surface) while attached to the parent silver iodide crystal. It is believed that this twinning is encouraged by the continuous supply of iodide ions to the growing (face-centered cubic) phase, either by bulk diffusion through the dispersing medium or by anionic diffusion through the crystal junction.
In general, one twinned face-centered cubic mixed halide crystal is formed at the single basal face of a hexagonal pyramidal silver iodide crystal, and two twinned mixed halide crystals are formed at the two basal faces of each hexagonal bipyramidal silver iodide crystal. FIG. 3 of No. 1596602 shows one of No. 1596602 hexagonal pyramidal silver iodide crystals (3a) and one hexagonal bipyramidal crystal (3b). As precipitation of the mixed silver halide is continued and the total iodide proportion of the silver halide suspended in the dispersion medium decreases to 30-40 mole % iodide, the dissolution of the originally formed silver iodide crystals becomes predominant and the `dumb-bell`-shaped crystals of FIG. 4 of No 1596602 are observed. FIG. 4 of this patent, shows one twinned face-centered cubic crystal formed on a hexagonal pyramidal silver iodide crystal (4a) and one twinned face-centered cubic crystal formed at each basal face of a hexagonal bipyramidal silver iodide crystal (4b). As step (b) proceeds the twinned face-centered-cubic mixed halide crystals increase in size and the iodide crystals decrease in size. This stage is shown in FIG. 5 of No 1596602. Eventually the silver iodide linkage between the two twinned crystals (5b) is broken and the two twinned crystals are released. The residue of the silver iodide remains initially on the twinned face-centered-cubic crystals but eventually dissolves away and is incorporated in the growth crystals.
FIG. 6 of No. 1596602 is an electron micrograph showing the dumb-bell crystals of FIG. 4b in the process of recrystallization.
In the process described in No. 1596602 the supply of iodide ions in step (b) hereinafter called the recrystallization step is provided by further dissolution of the silver iodide crystals to maintain the equilibrium concentration given by the relationship: EQU [Ag+][I-]=k
where [Ag+], [I-] are the activities (in dilute solution the concentrations) of silver and iodide ions, and k is a constant (k is the well-known solubility product).
As hereinbefore stated, the incorporation of iodide in the growing crystals in step (b) encourages the formation of octahedral faces, and in particular, the formation of stacking faults known as twin planes. It is known that the formation of twin planes is not possible when the external faces of the crystals are the cubic (100) lattice planes (Berry and Skillman, Photographic Science and Engineering 6, page 159 (1962)), but can occur only when the external faces comprise at least partially the octahedral (111) lattice planes. Thus the incorporation of iodide in the recrystallization step (b) has the effect of encouraging twin formation, even under conditions where, with crystals containing no iodide, cubic external faces would normally be displayed.
In step (b) as iodide ions are removed from the solution phase by precipitation, they are rapidly replaced by the dissolution of further silver iodide crystals, so that depending on the addition rates of the silver and halide solutions the silver iodide crystals are completely dissolved by the end of the precipitation or recrystallization step (b).